1. Field of the Invention
This invention relates to error detection in long distance digital data transmission lines during live traffic transmission and more specifically to apparatus and methods for providing channel performance testing without knowledge of the channel contents.
2. Description of Related Art
Digital telephony was introduced originally as a carrier for voice communications. Its purpose was to expand the capacity of transmission lines. Non-voice digital data transmission proliferated with the growth of data processing application of voice frequency (VF) analog equipment, including pulse code modulation (PCM) facilities. Direct digital customer data links have been prominent since the introduction of the Digital Data Service (DDS) in the early 1970s. DDS offers a maximum rate of 56 KB/sec. To accommodate higher traffic rates a full T1 channel (1.544 Megabits per second in the USA) is being offered by many carriers.
Various restrictions have been placed on the data introduced into these channels. They were leased originally subject only to a minimum "ones density" requirement and a maximum number of succesive zeros permissible in the data stream so that clock recovery circuits along the route would be refreshed before the recovered clock, which is the basis of digital data regeneration, had decayed or had drifted too far from nominal frequency. This proved to be unsatisfactory because, in the absence of any specified framing format for data, the carriers are unable to frame on the multiplicity of data formats used by various vendors and are thus unable prior to the present invention to verify customer complaints about channel errors. Recently certain carriers have attempted to solve this problem by adding a further format requirement to new service offerings. In particular a requirement of "D4" format has been specified. This means that every 193rd bit is required to be devoted to an overhead function, framing. Bell System Technical Journal dated November 1982 describes such D4 framing. Not only does this have the undesirable results of restricting the data rate and interfering with the functioning of much of the existing hardware, i.e., the equipment designed and installed by the industry to use the entire T1 channel, but it also places an undesireable restriction on encrypted data by providing an interceptor with someplace to start.
Also in the last few years a great deal of effort has been expended to permit channel performance monitoring during live traffic transmission. See AT&T Compatibility Bulletin 142, "The Extended Framing Format Interface Specification". The Extended SuperFrame (ESF) technique has been widely promoted by the Bell System and others. The ESF format provides a limited cyclic redundancy code check (6 bit) on live traffic. It also provides a 4 Kilobit per second supervisory or maintenance channel. A great deal of hardware is being developed for ESF, some of it is now starting to be deployed.